Fix margin winner_token in margin_data and extract _classify_stability helper

The Decision Stability display was showing the sampled token as the winner
rather than the logit-top token. When temperature changed the pick (e.g.
"(number" was logit winner but "(n" was sampled), both sides showed the same
token. Now includes winner_token in margin_data from logits_entries[0].

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/model_service.py

@@ -2890,8 +2890,11 @@ async def analyze_research_attention_stream(request: Dict[str, Any], authenticat
                     # Sampling sensitivity: did temperature change the outcome?
                     sampling_sensitive = next_token_id != greedy_token_id
 
+                    winner_token = logits_entries[0]["token"] if len(logits_entries) > 0 else ""
+
                     margin_data = {
                         "margin": margin,
+                        "winner_token": winner_token,
                         "winner_logit": winner_logit,
                         "runnerup_logit": runnerup_logit,
                         "runnerup_token": runnerup_token,